Anguillid Eels as a Model Species for Understanding Endocrinological Influences on the Onset of Spawning Migration of Fishes

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations

Eel Cruise XVIII

航海番号: KH-12-2 Leg 1, Leg2 ; 航海日程: May 13 - June 1, 2012 ／ June 6 - June 28, 201

Profiles of thyroid hormones and mRNA expression for thyroid stimulating hormone in Japanese eel during downstream migration

To understand the roles of thyroid hormones in downstream migration of anguillid eels, we examined the profiles of serum thyroid hormones (thyroxine, T4 and triiodothyronine, T3) and thyroid stimulating hormone β-subunit (TSHβ) mRNA expression in relation to habitat use, silvering, and seasonal changes. Female Japanese eels, Anguilla japonica, were collected in the brackish Hamana Lake and its freshwater rivers from July to December. The habitat use history of the eels was determined by otolith analysis of strontium and calcium concentrations. Thyroid hormone levels were measured by enzyme immunoassay and expression levels of TSHβ mRNA were determined by real-time quantitative PCR analysis. There were no consistent differences in the thyroid hormones or TSHβ expression levels between different habitats or seasonality. In addition, there were no or weak significant differences in thyroid hormones levels and TSHβ mRNA expression levels between migrant and other eels, although thyroid hormones have been shown to regulate endocrine control of downstream migratory behavior in some fishes. These findings suggest that thyroid hormones have play a minor role in downstream migration of anguillid eels

Eel Cruise XVIII

航海番号: KH-12-2 Leg 1, Leg2 ; 航海日程: May 13 - June 1, 2012 ／ June 6 - June 28, 201

Age and body size of Japanese eels, Anguilla japonica, at the silver-stage in the Hamana Lake system, Japan

Effects of salinity-different habitats on growth rate, age and body size of Japanese eels at the silver-stage were investigated. Fish were sampled in one drainage system, that includes the brackish Hamana Lake and its inlet freshwater rivers, and were evaluated using otolith strontium:calcium (Sr : Ca) analysis to determine whether the eels had been living in estuarine or freshwater habitats. The growth rate of estuarine eels was found to be significantly higher than that of river eels for both females and males. In male silver eels, there was no difference in age between estuarine and river eels, but the total length and body mass (BM) of estuarine eels were significantly larger than river eels. In female silver eels, the age of river eels was significantly older than those of estuarine eels and the BM of river eels was significantly greater than in estuarine eels. These results indicate that effects of habitat use and salinity differences on age and body size at the silver-stage were different between female and male eels in this system

Age and body size of Japanese eels, Anguilla japonica, at the silver-stage in the Hamana Lake system, Japan

Effects of salinity-different habitats on growth rate, age and body size of Japanese eels at the silver-stage were investigated. Fish were sampled in one drainage system, that includes the brackish Hamana Lake and its inlet freshwater rivers, and were evaluated using otolith strontium:calcium (Sr : Ca) analysis to determine whether the eels had been living in estuarine or freshwater habitats. The growth rate of estuarine eels was found to be significantly higher than that of river eels for both females and males. In male silver eels, there was no difference in age between estuarine and river eels, but the total length and body mass (BM) of estuarine eels were significantly larger than river eels. In female silver eels, the age of river eels was significantly older than those of estuarine eels and the BM of river eels was significantly greater than in estuarine eels. These results indicate that effects of habitat use and salinity differences on age and body size at the silver-stage were different between female and male eels in this system